About 30,000 juvenile coho salmon may be doomed by the drought as Sonoma County streams shrink and become disconnected from the Russian River, trapping the young fish in pools that will dry up or degrade over the long, hot summer, experts say.

The parched conditions have appeared earlier this year than any other in the state’s current dry spell, and they could prove the deadliest in recent record to the imperiled coho, the focus of 14-year-old restoration effort costing millions of dollars.

“It’s grim. It’s going to be a rough year for the coho,” said Mariska Obedzinski, a fish biologist who coordinates the UC Cooperative Extension’s coho monitoring program. “They can’t get where they need to go.”

At the same time, another 50,000 coho juveniles, known as smolts, are due for release from the Don Clausen Fish Hatchery below Warm Springs Dam on Lake Sonoma and scientists are considering which streams will give the endangered fish the best chance of achieving their biological goal of reaching the Pacific Ocean this spring.

Two coho spawning streams — Porter and Pena creeks — are already cut off from the river. If no more rain falls, other tributaries, including Green Valley, Dutch Bill and Mill creeks, will likely go dry in spots, Obedzinski said.

The California Department of Fish and Wildlife is already planning rescue operations to save the smolts and younger fish in disconnected streams.

“We will act quickly,” said Eric Larson, fisheries program regional manager for the state agency.

Smolts could be retrieved from creeks and relocated to the Russian River, while the younger fish, hatched this year, would be moved upstream to areas with more water or taken to the Lake Sonoma fish hatchery, he said.

Fish and Wildlife recently rescued 23 smolts from Redwood Creek in Marin County and placed them at the hatchery.

Coho are one of three local anadromous species, along with chinook salmon and steelhead trout, that are born in fresh water, spend most of their lives in the ocean and return to fresh water to spawn.

A multiagency effort to save the Russian River coho began in 2001, when the fish were on the verge of extinction. The effort now revolves around the annual release of about 200,000 hatchery-bred fish, planted in up to 20 tributary streams each year.

The springtime migration of coho smolts to the ocean began last month and will peak in late April or early May, setting up a race against time for the year-old fish, five to six inches long, to reach salt water.

“They’ve got to get out now,” said Bob Coey, a National Marine Fisheries Service biologist, underscoring the urgency of the situation.

“It’s challenging,” said Ben White, a fish biologist who runs the Don Clausen hatchery for the U.S. Army Corps of Engineers.

“We are trying to restore wild runs of salmon,” he said, including streams that are vulnerable to droughts.

Last spring presented a similar scenario, except that most coho streams became disconnected from the river in late May, trapping smolts toward the end of the run, Obedzinski said.

The current situation has presented itself earlier in the year and may be unprecedented, with the prospect of losing the majority of smolts in some streams, she said.

She said it was possibly the worst year for the fish since stream monitoring began in 2005.

The last pulse of rain, which dropped 1.75 inches in the hills west of Healdsburg on April 6 and 7, made a noticeable, if short-lived difference. Porter Creek opened for a few hours and “a lot of fish moved out,” Obedzinski said.

Smolts that make it to the ocean this spring will return to spawn in the winter of 2016-17, making a one-way run up the creeks where they were born or planted.

Up to 500,000 coho spawned in Northern California river systems in the 1940s, dwindling to about 5,000 by the mid-1990s, when scientists realized the need for human intervention, ultimately resulting in the hatchery-based coho salmon recovery program.

In addition, the Sonoma County Water Agency, the region’s main drinking water supplier, is completing a $12 million upgrade at its seasonal dam and diversion site on the Russian River to improve fish passage.

The agency has also spent roughly $8 million on the first stage of an ambitious project to rehabilitate six miles of habitat for coho and chinook salmon and steelhead in Dry Creek, which carries water released from Warm Springs Dam.

On Tuesday, Nick Bauer, a UC Cooperative Extension biologist, found 50 coho smolts in a fish trap on Mill Creek near Healdsburg, after finding 500 there on Saturday. “It’s a sign that fish are moving,” he said. “The sooner they do that, the better their odds are.”

Decades ago, coho spawned in 39 Russian River tributaries, he said.

Juvenile cohos have been spotted in as many as 18 streams in recent summertime surveys, but only four streams had fish last year, Bauer said.

There’s no definitive count of the number of coho in the creeks now, experts said. The mortality rate is naturally high among the fish released from the hatchery each year, Obedzinski said.

No one wants to plant fish in a stream that will soon dry up, said Mike Dillabough, chief of the operations and readiness division for the Army Corps’ San Francisco District. Officials from the agencies involved in the coho recovery program will meet Friday to consider the options.

Among the alternatives, White said, are releasing smolts to Mark West Creek, which tends to run naturally year-round, or to Dry Creek, which carries water from Lake Sonoma to the Russian River every day for delivery to the Sonoma County Water Agency’s 660,000 customers in Sonoma and Marin counties.

“You need to have a backup plan for a program like ours,” White said.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.